The increasing sophistication of electronic circuits and systems presents unique challenges for circuit designers. The operating frequency of modern electrical and electronic equipment continues to increase, in order to reduce the physical size and weight of the electronic circuits and systems. However, the trend is hindered by the possible generation of undesirable effects, such as ringing and resonance, due to parasitic effects associated with the components, the physical orientation of the components, and/or the layout of components, devices and/or conductive tracks on printed circuit boards within an electronic circuit or system. These parasitic elements degrade the high-frequency performance of the entire electronic circuit or system.
In electronic circuits and systems, parasitic capacitance is the unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. All actual circuit elements such as inductors, diodes, and transistors have internal capacitance, which can cause their behavior to depart from that of an ‘ideal’ circuit element. In addition, parasitic capacitance can exist between closely spaced conductors, such as wires or printed circuit board traces.
For example, an inductor often acts as though it includes a parallel capacitor, because of its closely spaced windings. When a potential difference exists across the coil, wires lying adjacent to each other at different potentials are affected by each other's electric field. They act like the plates of a capacitor, and store charge. Any change in the voltage across the coil requires extra current to charge and discharge these small ‘capacitors’. When the voltage doesn't change very quickly, as in low frequency circuits, the extra current is usually negligible, but when the voltage is changing quickly the extra current is large and can dominate the operation of the circuit.
The parasitic capacitance between the base and collector of transistors and other active devices is a major factor limiting their high frequency performance. The screen grid was added to vacuum tubes in the 1930s to reduce parasitic capacitance between the control grid and the plate, and resulted in a great increase in operating frequency.
An inductor is one of the key components in the input and output filters of an electronic circuit or system. It is typically used as the series element to attenuate undesirable signals. However, its equivalent parallel capacitance (EPC) and equivalent parallel resistance (EPR) significantly affect the inductor's high frequency (HF) performance, causing non-ideal filter behavior.
FIG. 1 shows the schematic representation of a prior art high-frequency model for an inductor 10 comprising the inductor and its EPC 12 and EPR 14. The inductor 10 behaves like a capacitor when the operating frequency is higher than the damped resonance frequency of the inductor. The damped resonance frequency is determined by the inductance of the inductor, its EPC, and its EPR. FIG. 2 shows an impedance against operating frequency curve 20 for a 470 μH inductor, in which its EPC is 1 nF and its EPR is 100 kΩ. The damped resonance frequency fdr of the inductor coincides with the peak value of impedance as illustrated in FIG. 2. The inductor impedance is dominated by the EPC at high frequencies and its impedance decreases with the operating frequency.
The EPC and EPR of the inductor will introduce undesired noise at the output of the filter, conducted noise at the input of the filter, and resonance with other components and parasitic elements in the circuit. FIG. 3 shows a buck converter 30 with a prior art inductor 10. The supply source 32 of the converter is vin. The duty time of the switching element S 34 is adjusted to control the output voltage across the load 36. FIG. 4 depicts the effects of EPC and EPR on the output voltage of the buck converter 30. Due to the parasitic capacitance of the inductor 10, there appear voltage spikes 40 at the output which coincide with the leading and trailing edges 42, 44 of the gate signal 46 for controlling the main switch of the buck converter. When the main switching element 34 is on or off, the voltage across the diode 38 is changed. Thus, voltage spikes will then be coupled to the output through EPC. In order to eliminate such noise, it is crucial to cancel the effects caused by at least the EPC and preferably also the EPR.
There are two main approaches to canceling the effects of parasitic capacitances in a circuit or system. The first is based on canceling the parasitic capacitance of a particular component while the second one is based on canceling the effect caused by all parasitic capacitances in the entire circuit or system.
Some coupled magnetic windings are used to nullify the effect of the parasitic capacitance of the inductor. The method is shown in FIG. 5, in which a negative capacitance is developed by configuring an inductor 10 as two inversely or opposed coupled windings 10a, 10b having different number of turns and connecting a capacitor 31 to a center tap between the two windings 10a, 10b. Although the EPC can be canceled, the structure will still produce an inductance in series with the added capacitor 31. Moreover, it cannot cancel the EPR effect and the coupling effect is highly dependent on the magnetic properties of the core materials of the coupled windings 10a, 10b. There are many disadvantages to this approach of addressing the problem of parasitic capacitance.
A prior-art method using the second approach is based on adding an active circuit. The parasitic effects are canceled at the circuit level, but this approach also suffers many problems and does not fully mitigate or obviate the parasitic capacitance effects.